Regarding image forming apparatus based on electrophotographic technology, following two systems have been known; one is a one-component developing system wherein only toner is employed as a developer for developing an electrostatic latent image formed on the image carrier, and the other is a two-component developing system wherein toner and carrier are used.
The one-component developing system generally uses a toner supporting member and a regulating plate pressed against the toner supporting member. While the toner on the toner supporting member is pressed by the regulating plate, film thickness is regulated, whereby forming a toner thin layer having a predetermined amount of electrostatic charge. The electrostatic latent image on the image carrier is developed by this toner thin layer. This system is characterized by excellent dot reproducibility, and easily provides a uniform image with the minimum irregularity. This system is also considered to simplify and downsize the apparatus, and to reduce the costs. However, a heavy stress is applied to the toner by the regulating section. This may degenerate the toner surface. Further, toner or external additive may stick to a toner regulating member and the toner supporting member surface, or may reduce the electrostatic charge of the toner. Fogging on the image due to poorly charged toner or internal contamination due to scattering with those toner will occur, with the result that the service life of the developing unit is reduced.
In the two-component developing system, electrostatic charge is caused by turiboelectric charging resulting from mixture of toner and carrier. This reduces stress and deterioration of toner. Due to its large surface area, the carrier that causes electrostatic charge of toner is relatively resistant to the contamination by toner or external additive, and hence, ensures a longer service life.
However, even when the two-component developer is used, carrier surface is contaminated by toner or external additive all the same. The electrostatic charge of toner will be reduced by a long-term use, and the problems involving fogging or scattering of toner will arise. Thus, the service life cannot be said to be sufficient. Some means must be provided to ensure longer service life.
In an effort to prolong the service life of the two-component developer, a developing unit is disclosed in the Unexamined Japanese Patent Application Publication No. S59-100471, wherein a carrier, together with toner or independently, is replenished little by little, and the developer of deteriorated electrostatic charge is ejected accordingly. The carrier is replaced, whereby the percentage of the deteriorated carrier is reduced. Through replacement of carrier, this device ensures that reduction in the electrostatic charge of toner due to deterioration of the carrier is kept to a predetermined level. This arrangement contributes to a longer service life.
Unexamined Japanese Patent Application Publication No. 2003-215855 discloses a two-component developer made up of the toner provided with external addition of the particles having a polarity of electrostatic charge reverse to that of the toner, and a carrier. The particles having reverse polarity in the development method based thereon serve as abrasive powder and spacer particles, and are effective in removing spent matters from the carrier surface. Accordingly, it has an advantage of reducing the possible deterioration of the carrier.
Unexamined Japanese Patent Application Publication No. H9-185247 discloses so called hybrid development method for developing a latent image on the image carrier by using the toner supporting member that supports only the toner from the two-component developer. The hybrid development method provides excellent dot reproducibility and image uniformity without a brush mark of the image being caused by a magnetic brush. Further, due to lack of direct contact between the image carrier and the magnetic brush, this method causes no transfer of the carrier to the carrier (consumption of carrier). This is an advantage that cannot be found in the conventional two-component developing systems. In the hybrid development method, toner is charged by triboelectric charging with the carrier. Accordingly, keeping of the charge applying property of the carrier is important for stabilizing the electrostatic charge of the toner and ensuring a long-term maintenance of image quality.
However, according to the Unexamined Japanese Patent Application Publication No. S59-100471, such problems as cost and environmental issues arise since a mechanism for collecting the ejected carrier, or the carrier gets to belong to consumable supplies. Further, printing of a predetermined number of sheets must be completed before the radio of a new carrier to the old is stabilized, and the initial characteristics cannot always be maintained. Moreover, the Unexamined Japanese Patent Application Publications Nos. 2003-215855 and H9-185247 involve the problem wherein, with the increase in the number of prints, the carrier surface is contaminated by toner or finishing agents, with the result that the charge-applying property of the toner is reduced.